The sole of an athletic shoe used in various sports is generally comprised of a midsole and an outsole fitted under the midsole, directly contacting with the ground. The midsole is typically formed of soft elastic material in order to ensure adequate cushioning properties.
Running stability as well as adequate cushioning properties are required in athletic shoes. There is a need to prevent shoes from being deformed excessively in the lateral or transverse direction when contacting with the ground.
As shown in Japanese Utility Model Examined Publication No. 61-6804, the applicant of the present invention proposes a midsole assembly having a corrugated sheet therein, which can prevent such an excessive lateral deformation of shoes.
The midsole assembly shown in the above publication incorporates a corrugated sheet in a heel portion of a midsole, which can produce resistant force preventing the heel portion of a midsole from being deformed laterally or transversely when a shoe contacts with the ground. Thus, the transverse deformation of the heel portion of a shoe is prevented.
However, it depends on the kind of athletics or athletes whether an athlete lands on the ground more frequently from the medial portion or the lateral portion of the heel at the onset of landing. For example, since tennis or basketball players move more often in the transverse direction and the medial portions of their heels tend to first contact the ground, the heels lean outwardly and so-called supination often occurs. On the other hand, since runners or joggers tend to land on the ground from the lateral portions of their heels and the load moves toward the toes, the heels lean inwardly and so-called pronation often occurs.
These pronation and supination movements are normal movements when an athlete's foot comes in contact with the ground. But over-pronation or over-supination may cause injuries to the ankle, knee and hip of an athlete.
In the conventional midsole design, there is provided a corrugated sheet having a constant wave configuration in both the transverse direction and the longitudinal direction of the heel portion. Therefore, the prior art midsole has a constant compressive hardness throughout the midsole and as a result, it cannot control effectively pronation and supination of the foot of an athlete although controlling them is required according to the kind of athletics.
Generally, by inserting a corrugated sheet, the heel portion of a midsole tends to be less deformed in the transverse direction. When the corrugated sheet is formed of high elastic material, the heel portion of a midsole tends to be less deformed in the vertical direction as well. Therefore, when the corrugated sheet has a constant wave configuration, the heel portion of a midsole where adequate cushioning is required may show less cushioning properties when contacting with the ground.
On the other hand, good cushioning properties are indispensable requirements of athletic shoes, but too high cushioning properties may absorb the athletic power such as the running or jumping power of an athlete.
The object of the present invention is to provide a midsole assembly for an athletic shoe that can prevent the over-pronation and over-supination on landing by preventing the shoe from being deformed in the transverse direction according to the kind of athletics, and not only ensures adequate cushioning properties but also prevents an athletic power from being lessened.